1. Field of the Invention
The present invention relates to a tree structure extraction apparatus, method and program for constructing tree structures of specific structures detected in image data. Particularly, the present invention relates to a tree structure extraction apparatus, method and program for constructing tree structures of plural specific structures located close to each other, for example, such as a portal vein and hepatic veins in a liver, respectively. The portal vein and the hepatic veins in the liver run close to each other as if branches of blood vessels of the portal vein and those of the hepatic veins are entangled or intermingled with each other.
2. Description of the Related Art
When a surgery is performed on an organ, such as a liver and a lung, to remove a diseased part therefrom, for example, if the organ is a liver, blood vessels, a hepatic parenchyma, and a tumor region need to be extracted from an X-ray CT image of the liver. Further, a blood vessel dominating the extracted tumor region needs to be identified based on the positions of core lines, diameters and the like of the extracted blood vessels. In this way, the blood vessel that supplies nutrition to the tumor should be identified, and a region dominated by the blood vessel should be appropriately determined, as a region to be removed. Such processes are necessary to appropriately remove a part of a portal vein supplying nutrition to the tumor and a region that is dominated by the part of the portal vein, and into which a substance to be noted, such as cancer cells, may have spread, in such a manner that the function of the liver is maintained even after removal. Therefore, it is important to precisely simulate a region to be removed before the surgery. Further, it is necessary to accurately extract the center lines of blood vessels running in a lung or a liver for the simulation operation.
Here, a method using a Hessian matrix has been proposed, as an image recognition technique for extracting a linear structure, such as bronchi, from a three-dimensional medical image obtained by CT or the like. Specifically, first, multiple resolution transformation is performed on the three-dimensional medical image. Then, an eigenvalue analysis of Hessian matrix is performed on an image of each resolution, and linear structure elements are extracted. The linear structure element has a characteristic that only one of three eigenvalues obtained by eigenvalue analyses is close to 0. Then, analysis results about images of respective resolutions are unified to extract linear structure elements (blood vessels) in various sizes from the three-dimensional medical image. Further, the extracted linear structure elements are connected to each other by using minimum spanning tree algorithm or the like. Consequently, data of a tree structure representing a tubular structure in the three-dimensional medical image are obtained. When the linear structure elements are connected to each other by using minimum spanning tree algorithm, a cost function based on a positional relationship between the linear structure elements and the principal axis direction of each of the linear structure elements represented by an eigenvector corresponding to an eigenvalue close to 0 is used (U.S. Patent Application Publication No. 20110135172 (Patent Document 1)).
However, when the number of times of branching is large and branches of blood vessels run close to each other as if they are entangled with each other, as in a portal vein and hepatic veins in a liver, if blood vessels such as the portal vein are extracted by using a technique similar to Patent Document 1, a branch of a hepatic vein, which is not the portal vein, is erroneously extracted as a branch of the portal vein in some cases.